In semiconductor manufacturing and, in particular, for forming metal layers or interconnects, the metal is typically electrodeposited on a metallic seed or plating base layer. The seed layer is typically formed on a semiconductor wafer by physical vapor deposition (PVD) or chemical vapor deposition (CVD). The current flow driving the electrodeposition is passed laterally through the seed layer from the electrical contact established at the seed layer edges. Current through the semiconductor wafer itself and any dielectric layers contained within the semiconductor wafer is essentially negligible.
Electrochemical processing of semiconducting/insulating materials can occur where the semiconductor wafer, typically a thin oxide coating or semiconductor, can conduct small amounts of an electric current. Conductance can be provided by transport of charges directly through the valence or conduction band of a semiconducting/insulating semiconductor wafer. The latter case becomes important when working with semiconducting oxides such as TiO2, ZnO and Ta2O5, as well as any n-type wide band-gap semiconductor. For thin oxide/insulating layers, electron tunneling or current leakage can also be a mechanism for conductivity. Defects can also provide many electron states in the band gap of an insulator and for a high density of gap states these defects can provide a pathway for the electric current.
This principle is illustrated schematically in FIG. 1. When empty oxidant states in solution overlap with the electron states of a conduction band of a semiconductor (FIG. 1A), electroreduction of oxidants will be possible at an n-doped semiconductor wafer (conduction band electrons available) but not at a p-doped semiconductor wafer (no conduction band electrons available in the dark). If the Gaussian distribution of oxidant states corresponds with energies in the band gap of a semiconductor, no charge transport will be possible and the oxidant can be expected to be inactive at this particular semiconductor wafer (FIG. 1B). However, if oxidant states in solution overlap with the valence band of a semiconductor, charge transfer is possible both for n-doped as well as p-doped semiconductor wafers (FIG. 1C). In sum, the electronic structure of the insulator and semiconductor semiconductor wafer in combination with the electronic structure of the redox electrolyte in solution allows for only certain charge transfer combinations.